I noticed that the soundtrack is on Steam. Can you put it on itch? You have the soundtrack for the first game on here but not on Steam oddly enough.
I don't know if this covers your desire here, but you can set a download to only be available to people who have paid a certain amount (for example, $2 gets you the game, $4 gets you the game and the soundtrack). I don't know how that interacts with the mega-bundle though.